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Mason Flats Wetland Enhancement Project

In early 2013, Environmental Services completed a stormwater management and wetland enhancement project at NE Mason and Airport Way in the Big Four Corners Natural area. The project site encompasses two waterways and a historic wetland that was farmed for some time, then became, in part, a private wetland mitigation site.

Project Details

The Mason Flats Wetland Enhancement Project reduces pollutants in stormwater that drains from more than 600 acres of upland development and roads. This also reduces pollutants that entering the Columbia Slough. The wetland was graded to provide additional water flow, especially the cold spring water from up slope areas to the south. However, the ponds in the eastern portion of the site were not changed as they already provide healthy amphibian and turtle habitat.

Habitat enhancement included reducing invasive species like reed canarygrass and increasing the diversity and quantity of native vegetation. This increases native wetland habitat for sensitive wildlife species, especially red-legged frogs, Western painted turtles, and willow flycatchers and other neo-tropical migratory birds.

Increased tree canopy in the summer will also help cool the slough. The project also increases off-channel floodplain and flood storage in the former floodplain of the Columbia River where more than 90% of wetlands have been filled or developed.

The project placed numerous standing logs, root wads, and rocks to increase habitat for sensitive wildlife species, especially redlegged frogs, native fishes, birds and turtles. More shrubs will especially benefit willow flycatchers and yellow warblers. Several existing ponds and their unique aquatic vegetation remain untouched. Beavers are active on the site and beaver dam construction has been continuous in recent years. The effects of beaver dam building provide additional benefits in this wetland area.

Mason Flats is also providing new nesting habitat for turtles. Two long south-facing slopes and additional mounds of gravelly soil have been designed for turtle nesting. Western Painted turtles use ponds in the eastern portion of the site and the gravel and sun-warmed sites are preferred and necessary for egg incubation.

Mason Flats has been partially planted with grass, willow cuttings and shrubs. It already looks fabulous. Multiple benefits from projects like these are called for under Portland’s Watershed Management Plan and the City’s Migratory Bird Act responsibilities.